Recently, high-power alkaline storage batteries such as nickel-metal hydride storage batteries and nickel-cadmium storage batteries have been used as power sources for electric vehicles, hybrid vehicles, power-assisted bicycles, electric power tools and so on.
As a separator to be used in an alkaline storage battery, for example a nonwoven fabric composed of polyolefin-based fibers excellent in alkali resistance is used. However, polyolefin-based fibers are poor in affinity with an alkaline electrolyte solution, and is therefore required to be subjected to a hydrophilic treatment to improve hydrophilicity. Thus, the nonwoven fabric composed of polyolefin-based fibers is used as a separator for an alkaline storage battery after being subjected to various hydrophilic treatments such as a sulfonation treatment, a fluorine gas treatment, a corona discharge treatment and a graft polymerization treatment.
A separator into which sulfone groups are introduced by a sulfonation treatment or the like is known to suppress self discharge and contribute to capacity retention by trapping nitrogen-based impurities (ammonium salts) remaining in a positive active material.
However, the sulfonation treatment for introducing sulfone groups is complicated and takes time because a separator is immersed in concentrated sulfuric acid, then neutralized, washed with a large amount of water, and dried. Therefore, a separator into which sulfone groups are introduced by a sulfonation treatment or the like is produced at a high production cost.
When a sulfonation treatment is performed, carbon-carbon bonds in polyolefin-based fibers are broken, and therefore the mechanical strength of a separator having sulfone groups is easily reduced.
Further, it is difficult to sufficiently sulfonate the insides of fibers even by a sulfonation treatment, and therefore a separator having sulfone groups has low liquid retainability and is poor in cycle life performance and high rate discharge performance (JP-A-2005-310625).